This invention relates to a semiconductor integrated circuit containing a Hall-cell and more particularly to an integrated ion-implanted Hall-cell.
Integrated Hall-cells are typically formed in an isolated pocket region of an epitaxial layer having been grown over a silicon substrate. The substrate is usually of P-type conductivity while the epitaxial layer is N-type, having been uniformly doped during the epitaxial growth step. A second pair of ohmic contacts on the surface define an axis that is perpendicular to that defined by the first pair of current-passing contacts. A Hall-voltage V.sub.H is generated between the second pair of contacts which voltage is proportional to a magnetic field B that is perpendicular to both of the above noted axes in accordance with the equation EQU V.sub.H =B.mu..sub.H (.rho./t)I (1)
where .mu.H is the mobility of majority carriers in the Hall-cell body (the epitaxial layer), .rho. is the resistivity of the body and t is the thickness of the body.
When the Hall-cell is to be operated with a fixed voltage V across the first pair of contacts, Equation (1) is more conveniently changed to EQU V.sub.H =B.mu..sub.H (W/L)V (2)
where W is the distance between the contacts of the second pair and L is the distance between the contacts of the first pair.
The magnitude and precision of the thickness t and resistivity .rho. of the Hall-cell is determined during the epitaxial growth step. The range of values which can be used for .rho. and t of the epitaxial layer are constrained by the requirement for other integrated circuit components to the approximate limits of 0.3 to 6.0 ohm-cm. and 5 to 16 microns, respectively. Specific device requirements such as transistor breakdown voltage, transistor saturation voltage, epitaxial resistor value or epitaxial resistor temperature coefficient often restrict the usuable epitaxial layer parameters to the more limited range of 1-3 ohm-cm and 8-14 microns thickness.
In volume manufacturing, typical overall tolerances for the resistivity and thickness of the epitaxial layer are +15% for each parameter. Thus, in the case of operation at constant current I, it can be seen from the above equation that the Hall-cell voltage which depends upon .rho./t can vary as much as .+-.30%. On the other hand, in operating with a fixed voltage applied between the first pair of contacts, the dependence of the Hall-cell voltage V.sub.H on epitaxial parameters is greatly reduced. However, in this case the power consumption of the Hall-cell can now vary .+-.30% which is unsatisfactory for many applications.
For a given epitaxial resistivity and power consumption, the Hall-cell sensitivity (or the magnitude of V.sub.H in a given magnetic field) increases as the thickness of the epitaxial layer is decreased. Thus for optimum Hall sensitivity it is desirable to reduce the thickness of the Hall-cell body. This can be accomplished by introducing P-type impurities from either the top or the bottom surface of the epitaxial layer to occupy a portion of the above said pocket. However, when the thickness of the Hall-cell body is reduced in this manner, the absolute tolerance on the thickness of the body is degraded. For example, if in a 10 micron thick epitaxial layer of N-type, a P-type buried layer occupies 2/3 of the epitaxial pocket, the unaltered N-type epitaxial layer portion becomes the Hall-cell body and has a thickness of 3.3 microns. The thickness of the epitaxial layer having an initial tolerance of .+-.15% leads to a tolerance on the thickness of the Hall-cell body of greater than .+-.45%. The resulting variation in power consumption or in Hall sensitivity will be unsatisfactory for most applications.
It is an object of this invention to provide an integrated semiconductor circuit having a Hall-cell in an isolated portion of an epitaxial layer, the parameters of the Hall-cell being independent of the resistivity and thickness of the epitaxial layer and thus capable of more precise control in manufacture.
It is another object of this invention to provide in an integrated circuit a Hall-cell with precisely controlled sensitivity and power consumption.
It is a further object of this invention to provide an integrated circuit with a precisely controlled Hall-cell capable of being formed by methods more compatible with and less restrictive of the method steps which may be used to form, and control the parameters of, other integrated circuit components such as resistors and transistors.
It is also an object of this invention to provide in an integrated circuit a Hall-cell and resistors, all having precisely controlled values of resistance and the same temperature coefficient of resistance.